In the photographic field, cameras which are generally referred to as flow cameras have been used principally to photographically reproduce large documents such as engineering drawings, maps and architectural plans. Normally, the reason for such reproductions is to reduce the size of the documents to make them more easily stored while retaining them in a form that allows their content to be recovered when needed. Industries such as aircraft and automotive manufacturing make use of large, roll length drawings and tracings, and the flow camera is desirable because it can reproduce this type of document in one piece on a roll of film.
The flow cameras that have been available in the past have suffered from a number of problems that have detracted from their widespread acceptance. Perhaps most notably, they have not been able to provide quality reproductions of documents that are of somewhat poor quality due to original poor quality or weak lines, age discoloration, dirt, seepage of adhesive from tape, or other reasons. In fact, the photographic process used by prior flow cameras frequently reduces the quality. Therefore, conventional flow cameras are not really useful for the large number of documents that are lacking in quality for any of a number of reasons.
Prior flow cameras provide lighting that is arranged to illuminate only the side of the original document that is located closest to the lens. This practice is not effective to reproduce material that may be on the opposite side of the document. For example, many engineering drawings use both sides of translucent media such as tracing paper or drafting mylar sheets. It is also common for architects to draw a building floor plan on one side of a drawing and piping or duct work on the other side. If only the front side of the sheet is illuminated, the material on the back is not picked up in the photoreproduction. Even if there is no copy on the back of the sheet, back lighting which is transmitted through the sheet at proper intensity can significantly sharpen lines on the front and thus enhance the image on the front in the photographic reproduction.
Because prior flow cameras for large documents have provided illumination on only one side, they have been able to use conveyor belts beneath the original copy. In contrast, if the copy is illuminated on both sides, belts and the like must be absent from the area where the copy is exposed, as belts would adversely affect the transmission of light even if they are transparent. Consequently, the concept of providing illumination on both sides of the original copy creates new problems with respect to designing a suitable conveyor system.
Devices commonly referred to as rotary microfilm cameras have been available for a number of years. Although the rotary camera lights the original copy on both sides, it does so in order to provide microfilm images of small, two sided documents such as checks. Rather than obtaining a single congruent image from the two sides of the document, the rotary camera obtains separate and independent images of the two sides which are typically stored side by side on the microfilm. The rotary camera does not have the capability of obtaining a congruent image of material on opposite sides of the same original document.
35 mm microfilm and aperture cards have also been used to store documents at a reduced size. However, the size reduction on microfilm is so severe that a significant reduction in quality occurs even if proper techniques and good equipment are used. Microfilm images are generally between 1/12 and 1/36 the size of the originals, and prints made from microfilm are poor in quality in all cases and unacceptable if the original is poor or even mediocre in quality.
In recent years, digital scanning processes have been developed which allow documents to be scanned and stored as retrievable digital data. Using digital techniques, central storage of all documents in one database is possible, and the data can be transmitted electronically and instantly over long distances. The scanning is carried out with solid state charge coupled devices (CCD) and employs up-to-date electronic technology and software. Despite the sophistication of scanning systems, they are inadequate if the original image quality is poor and also when parts of the image are on opposite sides of the sheet which is undergoing the scanning operation. Approximately 20% of original documents are not candidates for effective digital scanning because of their low quality.
Even though digital scanning processes are able to achieve enhancement of some of the images that are scanned, they do so at a considerable cost, both in terms of the time that is required and the involvement of technical personnel. While the scan time is usually less than a minute, it can take hours to raster edit and sample prior to the scan and to resample after the scan. At times, it is easier to have the document redrawn than it is to scan it with solid state sensors for image enhancement, and there is always a finite limit to the restoration that can be achieved digitally.
The complexity of digital image enhancement is due partly to the large number of functions that the enhancement should perform. These include thresholding, deskewing, edge sharpening, edge smoothing, image averaging, scaling, contrast enhancement, warping/rubber sheeting, speckle removal, gap jumping, hole filling, and scaling.
Although some of these functions are handled automatically by software, others require interactive operator participation which can involve significant expenditures of time. For example, thresholding techniques require that the operator exercise judgmental skills to determine the threshold point and select either absolute or dynamic threshold modes. If dynamic thresholding is selected, the operator must decide which of the available contrast settings will return the best raster file data. In actual practice, operators typically prompt the scanner to cover a section of the document, retreat by backing up, and then advance again, all the while watching a monitor to determine what effect the thresholding is having on the image. This forward and backward movement can be repeated until the operator is satisfied that the optimum image is obtained from the thresholding.
The problems of digitally scanning poor quality originals have been recognized. High quality industrial cameras of the type disclosed in U.S. Pat. Nos. 3,639,054 and 3,762,816 to Wally et al. have been used to photograph poor quality originals in order to enhance their readability and restore the image quality with modest size reduction in a range of four to eight times. Full size reproductions can be made from the enhanced image on the film, and the reproductions can be enlarged with successful results because the camera enhancement in the negative makes a good quality reproduction possible.
Nevertheless, this technique is disadvantageous in that it is an expensive and time consuming task to first make a film negative (typically 81/2.times.11 inches) and then blow it up to original size. Making the enlargement on a mylar drafting film is more expensive and time consuming than making the film negative. In addition, the problem of storing the enlargement aggravates an already difficult storage problem. Roll length drawings are even more troublesome because their length requires that they be photographed in pieces rather than continuously as a roll of film.